Pulsed nuclear magnetic resonance (NMR) measurements use bursts of electromagnetic fields (also called rf pulses) at a specific frequency (also called the operating frequency or rf frequency) to induce response echoes in the material under measurement. To obtain the maximum response to a particular element, the operating frequency should be at the “Larmor frequency” (also called the “sweet spot” of the instrument). The Larmor frequency is dependent upon local static magnetic fields, which may change during logging. For example, temperature changes or the accumulation of magnetic debris on the tool will affect the strength of the static magnetic field and influence logging measurements. Because the tool is calibrated relative to the Larmor frequency, changes in the static magnetic field must be accounted for, such as by changing the operating frequency of the pulses and the resonance frequency of the antenna.
Accordingly, two frequencies should be tuned for proper instrument operation: (1) the operating frequency should be set to the Larmor frequency taking into account local static fields and (2) the resonance frequency of the antenna should be set to the rf frequency.
Improper tuning of an NMR logging tool affects the porosity calibration in two distinct ways. First, the strength of the rf pulses in the formation is changed, which leads to a reduced precessing magnetization. Second, a given precessing magnetization will induce a smaller signal in the spectrometer.
The conventional method of tuning Schlumberger's CMR™ tool is described in commonly owned U.S. Pat. No. 5,451,873 to Freedman, et al. (the '873 Patent) (incorporated by reference herein in its entirety). In the method of the '873 Patent, the tool is positioned in front of a high porosity zone, preferably as a station stop and the rf (operating) frequency is estimated based on the temperature reading of the sonde. The antenna, which operates like a parallel-resonant LC circuit, is then tuned to this frequency by the so-called “tune word search task” (TWST). The TWST uses a continuous test signal at the operating frequency injected with constant current into the antenna. The tuning capacitor is then varied while monitoring the detected amplitude of a signal injected into a test loop located on the antenna. The value of capacitance that provides the largest amplitude is selected for the best tuning. The TWST has to be done before driving the antenna with the electromagnetic bursts.
With this antenna tuning, the rf frequency is then adjusted to the proper Larmor frequency at the sweet spot with the so-called “Larmor frequency search task” (LFST). According to LFST, the Larmor frequency is determined by taking a set of measurements of the relative echo signal amplitudes at different operating frequencies. This set of measurements is fitted to a predetermined response curve to obtain the frequency maximum amplitude. Essentially, the rf-frequency is varied to find the maximum NMR signal. This is time consuming and may require that the tool be stationary in the well.
The temperature of the magnet is then monitored during logging and the rf frequency is adjusted based on the known temperature coefficient of the magnet material. A Hall probe may also be used to measure the magnetic field strength inside the sonde and detect accumulation of magnetic debris on the magnets.
The LFST cannot be performed during a NMR measurement and, therefore, needs to be performed beforehand. Accordingly, all other factors that affect amplitude need to remain constant during the LFST.
The LFST is a slow procedure because, for every setting of rf frequency, the amplitude has to be measured with a good signal to noise ratio. This is not always practical. A formation of at least 10 pu is required to accomplish the tuning procedure in which the signal strength for seven (7) different rf frequencies is measured and the optimal rf frequency and corresponding antenna tuning is inferred. During this time, the porosity in front of the tool must be constant, requiring a station stop in most cases. However, in many logging environments, station stops are not possible. Further, in fluid sampling tools, the flowing fluid will not likely have a constant hydrogen index.
During a continuous log it is not possible to verify directly that the operating frequency is tracking any changes in the Larmor frequency. Predictable changes occur due to changes in the temperature of the magnets that create the static field. The change of the magnet's field with temperature is a known magnitude and so the operating frequency can be corrected approximately by a measure of the sonde temperature. However, a sudden change in the magnetic field caused by magnetic particles attracted and deposited on the magnets can cause unknown shifts in the Larmor frequency that invalidate the tool calibration. To attempt detection of such shifts the magnetic field is measured in the vicinity of the zone under measurement. But this determination is coarse and does not allow for quantitative correction of the shifts.
Accordingly, there is a need for a continuous measurement of any deviation of the operating frequency from the Larmor frequency during the NMR measurement to allow the operating frequency to track this deviation by means of a feedback loop and so maintain the instrument calibrated throughout the measurement.
There is presented herein an improved method to continuously tune NMR logging tools, which accounts for changes in static magnetic fields.